Why can things like optical illusions “trick” our brain, even when you know for a fact what the reality is? Why is “what your brain thinks” not equal to “what you know”?


Why can things like optical illusions “trick” our brain, even when you know for a fact what the reality is? Why is “what your brain thinks” not equal to “what you know”?

In: 18

Brains like to understand what they are seeing. If they don’t understand, they will try to form an understanding based on things it already understands. Our eyes see something, our brain interprets based on familiarity.

This is how we learn.

Imagine a baby who has only had liquids it’s whole life. The baby witnesses mom putting food in her mouth. Baby thinks, “well, I put bottle/breast in my mouth and get food, maybe if I put other things in my mouth, they will be food too.” This results in baby eating new foods. This also results in baby eating non-foods…

This general learning behavior follows us our whole lives.

Keep learning!

Our brain is HEAVILY optimized for pattern recognition and prediction. It happens without our even being aware of it.

So when something breaks set patterns that we’re used to, that pattern has already been recognized and predicted by our brains well before we get to the point of conciously pondering the issue.

A lot of our thinking happens without us “thinking” about it at the level of a conscious internal monologue. It’s the same place we get gut feelings we can’t explain because they happen below that monologue level.

Most of our perception is subconscious. Your brain is wired to interpret input in a specific way. You can train your conscious mind to override it, but you’re basically trying to overcome evolution.

Processing things isn’t always a direct “hands on” thing.
You see a lot of things, and process them without having to think because your brain kind of already naturally “filters” things in various ways. Like the blind spot on everyone’s retina a few inches from the face. It kind of adds post processing or filter that fills in the gaps so you’re not constantly using thought to do basic things.

This sometimes gets weird since people aren’t directly in charge of it, but it’s faster and wasn’t an issue for many situations. Optical illusions are an issue for pilots since there’s a lot of weird situations, like looking at bright lights against a dark featureless background.
Brain wasn’t built around figuring that out quickly and accurately.

A lot of body processes work in trade offs like this, like breathing or heart rate.

Answer: there’s multiple reasons your brain can be tricked. For illusions one reason is because your brain doesn’t “see” in real time…nor is reality “streaming “ live. What actually happens is your brain takes snapshots of moments and then fills in the blanks to complete the picture. Whatever you are seeing has already actually happened, it’s in the past. That’s one way slight of hand magic tricks work so well, your brain completes the picture but it didn’t have the necessary information to know what the trick was so it guesses incorrectly what the completed picture is supposed to be.

Someone called optical illusions–“Brain Failures.”. Forgot who to credit.

Your brain takes shortcuts with image processing that can be exposed with optical illusions.

Different illusions can reveal specific structures or processing that can be tricked.

Of course these short cuts the brain takes in everyday life are totally fine; only when given a specific illusion are issues revealed.

Some illusions are based on 90⁰ angles of the western world. The Ames Room and Ames Window as examples. These illusions don’t work as well when tested on people who live in tribal societies without many Right Angles.

This is a long but interesting link:


There’s a lot of stuff in our brain that is in no way under our conscious control.

Our brains also apparently use a lot of “hacks” to get the job done- evolution is a “good enough” system, not an intelligent designer. For example, there are blind spots in our eyes due to the way our retina is shaped, our brains just edit that out.

We can’t control how our optic system works. We can’t choose to ignore or change information moving from our eyes to our brains, so even when we “know” an optical illusion is impossible we don’t get to choose how our “hardware” processes the visual information.

Finding things that interact with our vision system in a weird way is how we invent new optical illusions

The real reason is that most optical illusions you know about are modern contrivances that never existed until recently. Evolution shaped our vision based on the world in which we evolved. In the pre-modern world, illusions of the sort that trick us were so rare that there was no significant benefit in having top-down over-rides for perception. Yet there are costs to doing so, like new sources of mistakes and possibly slowing down perception. A feature that costs more than it benefits will not be selected for.

There are some natural illusions, but most of these are rare or so obviously false that, again, not much reason to develop special neural circuitry to bother. Adaptations need to work well most of the time.. not be perfect, always-100% right pipelines to pure truth.

So while this video doesn’t directly discuss optical illusions (actually deals with sound, but also talks about vision and how the ‘FPS’ of our brains affects things), it does have some interesting things to say on the way we (and other creatures) perceive the world around us which is tangentially relevant to your question. It’s pretty wild.


Not really ELI5 either, but super interesting.